Multiscale Modeling of Cardiac Calcium Dynamics; How Ryanodine Receptors can induce Arrhythmias Project Summary: Cardiac calcium (Ca) dynamics will be investigated to elucidate molecular and ionic mechanisms of delayed afterdepolarizations (DADs), early afterdepolarizations (EADs) due to spontaneous Ca releases from the sarcoplasmic reticulum (SR), triggered activities, and thus ventricular fibrillation (VF) t the tissue level to provide theoretical bases for novel therapeutic strategies. We recently showed how EADs due to reactivation of the L-type Ca current can result in large gradients of refractoriness in tissue, creating a substrate for reentry. EADs are also initiated by spontaneous Ca releases. DADs are normally initiated by spontaneous Ca releases. To understand how EADs and DADs are caused at the ryanodine receptor (RyR) level and induce triggered activities at the tissue level, we use computer simulation and mathematical analysis of physiologically detailed models of Ca cycling and the action potential (AP). In this study we consider five scales 1) single RyR 2) Ca release unit (multiple channels) 3) subcell (array of Ca release units) 4) whole cell 5) tissue. The first half of this research is to establish link betwee single RyR channel properties and subcellular Ca dynamics. Recently, Zima et al showed that Ca release from the SR can be via Ca spark or leak depending on SR Ca load. We will show how RyR channel opening and closing rates, which are regulated by cytosolic and luminal Ca, change the probability to form a Ca spark or leak. We also know Ca waves occur more often when the Ca is overloaded. We will investigate how a single spontaneous Ca spark recruits neighboring Ca release units to initiate a Ca wave, how clusters of Ca sparks propagate or terminate. The second half of this research is to establish the link between subcellular Ca dynamics and tissue AP dynamics. First, we will address how subcellular Ca waves interact with AP to induce DADs and EADs. Then we will address how DADs and EADs interact with tissue properties to induce a triggered activity. Key questions we will address are 1) what is the critica size of a group of cells of DADs or EADs to overcome electrotonic source-sink mismatch to become a triggered activity under different physiological conditions and tissue geometry. 2) how an EAD or DAD from a single cell can cause a group of cells to simultaneously generate EADs or DADs to exceed the critical size required for propagation. 3) how susceptibility of triggered activity in tissue can be estimated from the RyR properties.